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Donald Trump's double threat to global free trade

Donald Trump's decision to impose tariffs on steel and aluminium is a game changer, not because this hasn't been done before - it has. But because he has done so on "national security" grounds - the one WTO rule that cannot be challenged.

All countries can take actions to guarantee their national security, but are only expected to do so at times of real crisis, like a real war.

But now in peacetime, President Trump has cited national security as a reason for tariffs. If others follow suit the escalation could be impossible to stop or reverse.

No wonder that Roberto Azevedo, the WTO's director-general, is worried: "Just the threat of a trade war is already damaging," he told the BBC.

"But if this escalation goes on, and on, and on, you may have something very, very damaging - even the possibility of a recession. And if you look back in history, you know, in the 1930s, that's exactly where it led.

"That is why I have been saying, 'stick to the rules, do things according to the rules', because if others don't like it, there are mechanisms to take care of that," he says.

But those mechanisms themselves are also now under threat. The WTO's appellate body has been solving problems for more than 20 years, and its decisions have to be accepted by all members of the WTO. There is no veto.